The Origin and Nature of Flash Weld Defects in Iron-Nickel Base Superalloys

The purpose of this investigation is to determine the nature and morphology of weld defects on the fracture surfaces of flash welded high temperature, high strength alloys. Emphasis is on the material-related phenomena instead of the more commonly studied welding process variables.

Samples were fractured in a slow bend test using three-point loading. The resulting fracture surfaces were examined in a scanning electron microscope, and elemental analysis of distinctive fracture features was determined using the ancillary energy dispersive x-ray system. When the fracture surface analyses were completely documented, metallographic sections through distinctive features were examined using a bench metallograph.

The base materials used in this study exhibited banded microstructures in which the particle/segregate bands were elongated primarily parallel to the longitudinal axis. The orientation between these banded microstructural features and a propagating crack influence the fracture path. This, in turn, determines the fracture surface appearance and weld ductility.

A dimpled appearance predominated on every fracture surface studied. One exception to the microvoid fracture mechanism is crack propagation through an entrapped oxide film which formed during flashing. A flat spot is created where fracture occurs by this means. Oxides of aluminum and titanium are the primary constituents of the defect which has this morphology. All other flat spots and streaks are caused by bands of high particle density which become preferential paths for fracture propagation. The total strain to fracture is very low in these regions even though fracture occurs by initiation and coalescence of microvoids. Thus, flat spots and streaks appear flat at low magnification but most exhibit dimples at high magnification. These dimples are considerably smaller than those on the adjacent low particle density fracture surface.

In conclusion, this investigation showed that the base material microstructure can exert a marked influence on the occurrence of flash weld defects.